An ongoing problem in integrated circuit packaging is bent leads. Leads are becoming smaller, which reduces mechanical strength to resist bending, and at the same time leads are being crowded closer together, which reduces the tolerance for displacement. Current surface mount packages may have more than 300 leads with a pitch of 0.010 inches or less. These small fragile leads are susceptible to damage at each handling step. Integrated circuits must be tested, packaged for shipping, removed from shipping packaging, mounted onto a printed circuit board, soldered, and perhaps removed and resoldered. Each of these handling steps subjects leads to potential bending. A bent lead may cause a circuit fault by shorting to an adjacent lead or by failing to connect properly to the printed circuit board.
In a typical plastic packaging process, a metal lead frame is wire bonded to an integrated circuit. Plastic is molded around the IC, leaving part of the lead frame external to the plastic package. Lead frames have a stop bar to prevent plastic flow between the leads during the plastic molding process. Several metal cutting and forming steps are then used to remove unwanted portions of the lead frame (a border and parts of the stop bar) and to form the leads to the proper shape. For background information on molding and lead forming, see Packaging, handbook number 240800, available from Intel Literature Sales, P.O. Box 7641, Mt. Prospect, Ill. 60056-7641.
The packaged IC is typically tested after lead forming. After shipment, the IC package is placed onto a printed circuit board by pick and place equipment and soldered to a printed circuit board.
Several alternative solutions to bent leads are currently in use or proposed. For example, one proposed solution is to place a band of epoxy around the perimeter of the lead frame. The epoxy is removed just before the IC is placed on a printed circuit board. This solution requires cutting and forming to be deferred until the pick and place operation which in turn requires testing before cutting and forming. Therefore, this proposed solution requires extensive changes to standard processing equipment such as pick and place equipment and test fixtures. In addition, since the epoxy is removed before soldering, it does not provide protection during soldering or during repair if the IC needs to be removed and resoldered.
Some solutions require an additional part. For example, various holding or guard rings are used to temporarily support the leads until the soldering step. These solutions still leave the leads vulnerable at the critical soldering step and offer no protection after soldering or for removal and resoldering. Other solutions require packaged integrated circuits to be mounted onto a separate carrier which is soldered or plugged into a printed circuit board. This adds significant expense.
Still another alternative is tape automated bonding (TAB). TAB typically requires unique manufacturing assembly processes. For example, instead of a solder re-flow process which solders all parts simultaneously, TAB typically requires individual part soldering using heat bars or other single part methods. In addition, typical TAB frames have relatively little metal in the connections so that they cannot remove heat from an integrated circuit as effectively as formed metal leads.
A solution is needed which minimizes any required changes to existing manufacturing processes and equipment. A solution is needed which is low cost and which does not require additional parts. A solution is needed which provides permanent lead support. The support can then benefit the soldering process and possible resoldering.